Several methodologies are available for the design of an RF power amplifier. Stripline techniques, lumped LC networks, an broadband transformers are most commonly used for impedance matching functions in such designs. Practical implementation of wideband designs is facilitated most effectively through the use of broadband transformers.
Broadband transformers can be categorized either as conventional coupled winding structures or as transmission line transformers. Although transmission line transformers are useable at high frequencies, balanced to unbalanced winding transformations are often required in addition to the intended impedance transformation in such realizations. These structures are physically large and fabrication becomes difficult and costly.
Conventional coupled winding transformers are capable of balanced to unbalanced transformation while simultaneously providing the required impedance transformmation, but are useable only at low operating freqencies. Their fundamental limitation lies in the control of parasitic reactive elements internal to the construction of the transformer.
Used as impedance matching elements, the input and output transformers are among the most critical components in the design of a multi-octave amplier. The total performance of the amplifier (linearity, efficiency, VSWR tolerance, gain flatness) will depend on their quality.
In the past transformers capable of operating with low secondary impedances and large transformation ratios have been constructed using sections of low-dielectric constant, semi-rigid coaxial cable. Although occupying reasonable volumes, the units are difficult to fabricate, and are limited in power handling capability due to the thermal isolation of the inner conductor. One factor here is that these structures achieve low-frequency operation by the addition of E & I core-type ferrite materials using shapes that add to the manufacturing cost.
One approach that has been an improvement over the foregoing approach has been to construct the transformer in a planar format. Although using low cost ferrite plates, the laminate dielectric material limits operation to low-power levels, typically under 10 watts. The spiral winding geometry and multiple-turn secondary also necessitates the use of discrete crossover metalization and insulators.
Therefore, it would be desirable to produce an improved broadband RF transformer.